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# What is the value of y^3 - t^3?

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What is the value of y^3 - t^3? [#permalink]

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23 Nov 2011, 09:57
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What is the value of y^3 - t^3?

(1) y - t = 3
(2) y^2 - t^2 = 21

Source: gmathacks.com
[Reveal] Spoiler: OA

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Re: What is the value of y^3 - t^3? [#permalink]

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23 Nov 2011, 11:31
1. + 2.
(y+t)(y-t)=21
3*(y+t)=21
Y+t=7 and y-t=3 so we can solve for both t and y.

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Re: What is the value of y^3 - t^3? [#permalink]

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13 Sep 2012, 12:55
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metallicafan wrote:
A fast way to solve this question? I solved it, but it took me more than 2 min :s.

What is the value of $$y^3 - t^3$$?
(1) $$y - t = 3$$
(2) $$y^2 - t^2 = 21$$

I spent a lot of time in solving the equations with the possible values of y and t.

Source: gmathacks.com

(1) - (y-t) = 3 ----> Y & t can be anything -Insufficient
(2) (y+t)(y-t) = 3 x 7 ----> (y+t) can be 3 or 7 -----> i.e. y will be 5 for sure but t can be 2 or -2 ----> Insufficient
(1+2) (y+t)= 7 because (y-t) = 3----> y = 5 & t=2--->Sufficient

Hope it helps.
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Re: What is the value of y^3 - t^3? [#permalink]

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13 Sep 2012, 13:39
y^3 - t^3 or x^3 - y^3 is the same if you are more comfortable

(x-y) * (x^2 - y^2) -----> (x - y) * (x - y) * (x + y)

1) x - y = 3 -----> 3 * 3 * (x + y) ----> we do no have the value od (x + Y)

2) we have 21 BUT not (x - y)

1) + 2) we have 3 * 21 ----> Sufficient

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Re: What is the value of y^3 - t^3? [#permalink]

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13 Sep 2012, 14:14
carcass wrote:
y^3 - t^3 or x^3 - y^3 is the same if you are more comfortable

(x-y) * (x^2 - y^2) -----> (x - y) * (x - y) * (x + y)

1) x - y = 3 -----> 3 * 3 * (x + y) ----> we do no have the value od (x + Y)

2) we have 21 BUT not (x - y)

1) + 2) we have 3 * 21 ----> Sufficient

Hi Carcass,

You are right that it does not make any difference between y,t & x,y.

But you made a mistake in the formula $$x^3-y^3$$ is not equal to $$(x-y)$$ * $$(x^2 - y^2)$$
Instead $$(x-y)^3$$ is equal to $$x^3-y^3$$ + $$3xy(x-y)$$

I Hope it helps.
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Re: What is the value of y^3 - t^3? [#permalink]

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13 Sep 2012, 16:08
fameatop wrote:
carcass wrote:
y^3 - t^3 or x^3 - y^3 is the same if you are more comfortable

(x-y) * (x^2 - y^2) -----> (x - y) * (x - y) * (x + y)

1) x - y = 3 -----> 3 * 3 * (x + y) ----> we do no have the value od (x + Y)

2) we have 21 BUT not (x - y)

1) + 2) we have 3 * 21 ----> Sufficient

Hi Carcass,

You are right that it does not make any difference between y,t & x,y.

But you made a mistake in the formula $$x^3-y^3$$ is not equal to $$(x-y)$$ * $$(x^2 - y^2)$$
Instead $$(x-y)^3$$ is equal to $$x^3-y^3$$ + $$3xy(x-y)$$

I Hope it helps.
...........oh lord, after a long day of work the gmat is even worse.

Some Mod could say how to attack this question properly AND in the faster manner
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Re: What is the value of y^3 - t^3? [#permalink]

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14 Sep 2012, 04:41
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What is the value of y^3 - t^3?

(1) y - t = 3. If $$y=3$$ and $$t=0$$, we have one value for $$y^3 - t^3$$ and for $$y=4$$ and $$t=1$$ another. Not sufficient.

(2) y^2 - t^2 = 21. The same here. Not sufficient.

(1)+(2) From (2) we have $$(y-t)(y+t)=21$$. Since from (1) $$y - t = 3$$, then $$3(y+t)=21$$ --> $$y+t=7$$. So we have two distinct linear equation with two unknowns: $$y+t=7$$ and $$y - t = 3$$, which means that we can solve for them and get the value of $$y^3 - t^3$$. Sufficient.

Hope it's clear.
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Re: What is the value of y^3 - t^3? [#permalink]

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10 Nov 2013, 20:29
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Re: What is the value of y^3 - t^3? [#permalink]

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17 Mar 2015, 10:22
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Re: What is the value of y^3 - t^3? [#permalink]

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20 Jul 2016, 18:58
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Re: What is the value of y^3 - t^3? [#permalink]

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17 Aug 2017, 19:22
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Re: What is the value of y^3 - t^3?   [#permalink] 17 Aug 2017, 19:22
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